The present invention relates to bits, methods, and systems for roller cone drilling, and specifically to sealing technology.
Background: Rotary Drilling
Oil wells and gas wells are drilled by a process of rotary drilling. In a conventional drill rig, as seen in FIG. 5 a drill bit 50 is mounted on the end of a drill string 52, made of many sections of drill pipe, which may be several miles long. At the surface a rotary drive turns the string, including the bit at the bottom of the hole, while drilling fluid (or xe2x80x9cmudxe2x80x9d) is pumped through the string by very powerful pumps 54.
The bit""s teeth must crush or cut rock, with the necessary forces supplied by the xe2x80x9cweight on bitxe2x80x9d (WOB) which presses the bit down into the rock, and by the torque applied at the rotary drive. While the WOB may in some cases be 100,000 pounds or more, the forces actually seen at the drill bit are not constant: the rock being cut may have harder and softer portions (and may break unevenly), and the drill string itself can oscillate in many different modes. Thus the drill bit must be able to operate for long periods under high stresses in a remote environment.
When the bit wears out or breaks during drilling, it must be brought up out of the hole. This requires a process called xe2x80x9ctrippingxe2x80x9d: a heavy hoist pulls the entire drill string out of the hole, in stages of (for example) about ninety feet at a time. After each stage of lifting, one xe2x80x9cstandxe2x80x9d of pipe is unscrewed and laid aside for reassembly (while the weight of the drill string is temporarily supported by another mechanism). Since the total weight of the drill string may be hundreds of tons, and the length of the drill string may be tens of thousands of feet, this is not a trivial job. One trip can require tens of hours and is a significant expense in the drilling budget. To resume drilling the entire process must be reversed. Thus the bit""s durability is very important, to minimize round trips for bit replacement during drilling.
Background: Drill Bits
One of the most important types of rotary drill bits commonly used in drilling for oil and gas is the roller cone bit, seen in FIG. 8. In such bits, a rotating cone 82 with teeth 84 on its outer surface is mounted on an arm 46 of the drill bit body. The arms 46 (typically three) extend downhole from the bit body, and each carries a spindle on which the cone is mounted with heavy-duty bearings. The support arms are roughly parallel to the drill string, but the spindles are angled to point radially inward and downhole.
As the drill bit rotates, the roller cones roll on the bottom of the hole. The weight-on-bit forces the downward pointing teeth of the rotating cones into the formation being drilled, applying a compressive stress which exceeds the yield stress of the formation, and thus inducing fractures. The resulting fragments are flushed away from the cutting face by a high flow of drilling fluid.
The drill string typically rotates at 150 rpm or so, and sometimes as high as 1000 rpm if a downhole motor is used, while the roller cones themselves typically rotate at a slightly higher rate. At this speed the roller cone bearings must each carry a very bumpy load which averages a few tens of thousands of pounds, with the instantaneous peak forces on the bearings several times larger than the average forces. This is a demanding task.
Background: Bearing Seals
In most applications where bearings are used, some type of seal, such as an elastomeric seal, is interposed between the bearings and the outside environment to keep lubricant around the bearings and to keep contamination out. In a rotary seal, where one surface rotates around another, some special considerations are important in the design of both the seal itself and the gland into which it is seated. For instance, the conventional wisdom is that an elastomeric seal of the O-ring type should be under compressive stress (never under tensile stress), and while there should be enough contact stress between the seal and the rotating surface to prevent leakage, the contact stress should be minimized to reduce friction and wear. Additionally, there should be enough room in the gland to allow for expansion under changing conditions but not excessive room which could allow the seal to twist or buckle. Additional information regarding seals can be found in Leonard J. Martini, Practical Seal Design, (1984) and in Seals and Sealing Handbook (4.ed. M. Brown 1995), both of which are hereby incorporated by reference.
The special demands of sealing the bearings of roller cone bits are particularly difficult. The drill bit is operating in an environment where the turbulent flow of drilling fluid, which is loaded with particulates of crushed rock, is being driven by hundreds of pump horsepower. The flow of mud from the drill string may also carry entrained abrasive fines. The mechanical structure around the seal is normally designed to limit direct impingement of high-velocity fluid flows on the seal itself, but some abrasive particulates will inevitably migrate into the seal location.
For sealing on a rock bit, an O-ring, or a derivative of O-ring, is typically used. As the bit is operated, the seal will inevitably wear and fail, allowing the abrasive drilling fluid to quickly destroy the bearings. Even though the seals may be physically small, their longevity is often a key limitation in the lifetime of an expensive drill bit. Improvement in roller cone bit seal technology is therefore a very important factor.
Additionally, since the bearings of a roller cone bit have to operate at temperatures from room temperature up to several hundred degrees Fahrenheit, the bearings must be designed with a significant running clearance. This clearance, combined with the vertical and lateral forces on the bit, may require some flexure in the seal. The seal material must be able to flex elastically within the range defined by normal runout of the bearings, while still excluding drilling fluids.
FIG. 6A shows schematically an example of a conventional O-ring seal installed in its equilibrium position. (This Figure shows a cross-section through the seal in its installed position.) In this example, the seal 60 sits in a groove 62 (referred to as a xe2x80x9cglandxe2x80x9d). The seal makes a sliding contact to a sealing surface, which in this example is a cylindrical journal 64. The O-ring, while it lasts, will prevent the drilling fluid on the left side of the drawing from contaminating the lubricant on the right side of the drawing.
Before it is installed, the O-ring has a circular cross-section. However, FIG. 6A shows that the O-ring in its installed state is squashed into a flattened oval shape. The seal performs its sealing function by exerting contact stress on the sealing surfaces.
The O-ring is most deformed in the center of the flattened part. FIG. 4A shows three sample compression profiles for a particular installation. (The central profile is for nominal compression, where the dimensions of the O-ring and the seal housing are exactly as expected; the upper profile is for maximum compression, where the O-ring is a maximum size and/or the seal gap is a minimum size according to the accepted tolerances and the bottom profile is for minimum compression.) In each of these three cases, it can be seen that the compression is highest in the approximate center of the O-ring""s cross-section.
FIG. 4B shows the force profile for the middle curve of FIG. 4A. The force is highest at the middle of the sealing area""s width, and is zero at the edges. Thus the xe2x80x9cexclusion point,xe2x80x9d where the seal has its maximum resistance to fluid incursion, is at the center of the sealing area. This is also true of the many modifications of elastomer O-ring seals which have been proposed.
Several fundamental problems have been identified with O-ring-type seals in a rock bit application, and can cause various types of failure in the seal.
One type of failure is shown in FIGS. 6B-D. Particles of abrasive materials (fines and sediments) will tend to accumulate as an abrasive mass 66 at the edge of the O-ring, as seen in the left portion of FIG. 6B. This abrasive mass will abrade the O-ring-type seal, as seen in FIG. 6C, until it eventually reduces the sealing area of the O-ring seal and causes failure, seen in FIG. 6D.
A second type of failure is demonstrated in FIG. 7. In this drawing, the lubrication of the seal is insufficient to prevent adhesion, causing adhesion of the seal to the surface across which it is moving. The successive adhesion and dehesion shown in this figure is known as the Schallamach effect. This adhesion problem leads to frictional heating, tearing and cracking of the seal.
Bits, Methods, and Systems for Drilling with Lip Seal in Roller Cone Bit
The present application describes a radically new sealing technology for use in roller cone type rock bits. Instead of a seal having a smooth contact (with the exclusion point therefore located away from the seal edge), the present application teaches that an elastomer seal which provides high sealing force at its edge should be used instead, such as the seal shown in FIG. 1. With such a seal, the line of contact along a relatively sharp edge of the sealing surface provides rigorous exclusion of the abrasive-loaded drilling fluid from the interface between the seal and the journal, with the accumulation of abrasive materials away from the sealing surface. (Thus such a seal is more analogous to a lip seal than to a conventional O-ring-type seal.) In the preferred embodiment, this seal design produces a force profile which is OPPOSITE to that of a conventional O-ring-type seal: the sealing force is maximal at the edge of the sealing surface, and minimal in the middle of the sealing surface.
The compressive pre-load in particular can maintain the dynamic seal against a pressure differential across opposite sides of the seal structure.
In the most preferred embodiments, both edges of the seal are configured as oppositely-directed lip seals. The use of two exclusion points in combination provides a tightly controlled flow of lubricant.
A notable feature of some embodiments (though not necessarily of all embodiments) is that the edge portion of the seal is actually under tensile stress in some dimensions, but it is preferred that it is pre-loaded so that it is in compression.
In some embodiments, the seal dimensions are chosen so that the inner diameter of the seal, before installation, would be larger than the outside diameter of the journal. In these embodiments the gland dimensions are chosen to provide inward compression of the seal""s diameter. This force transfer also helps to hold the outer diameter of the seal firmly against the inner diameter of the gland. Additionally, surface area differences between the back of the seal and the lip contact zone ensure that the seal will not rotate in the gland.
In some embodiments, the two lips of the seal define an interior cavity which retains some lubricant. This cavity helps to assure adequate lubrication in the context of a tightly controlled flow of lubricant.
The disclosed innovations, in various embodiments, can provide one or more of at least the following advantages:
Extending the life of seals in roller cone rock bits;
Eliminating seal failure as the limiting factor in roller cone rock bit lifetime;
Avoiding abrasive and adhesive modes of roller cone rock bit seal failure; and
Reducing heat generation at the seal interface.
Another advantage of the innovative seal design, in some embodiments, is that it can be retrofitted into existing journal and gland designs. However, it is believed to be preferable, though not necessary, to use a gentler chamfer on the journal to ease assembly of the more fragile lip seal onto the journal.